The exofs short symlink names, stored in struct exofs_i_info.i_data and
therefore contained in the exofs_inode_cache slab cache, need to be copied
to/from userspace.

cache object allocation:
    fs/exofs/super.c:
        exofs_alloc_inode(...):
            ...
            oi = kmem_cache_alloc(exofs_inode_cachep, GFP_KERNEL);
            ...
            return &oi->vfs_inode;

    fs/exofs/namei.c:
        exofs_symlink(...):
            ...
            inode->i_link = (char *)oi->i_data;

example usage trace:
    readlink_copy+0x43/0x70
    vfs_readlink+0x62/0x110
    SyS_readlinkat+0x100/0x130

    fs/namei.c:
        readlink_copy(..., link):
            ...
            copy_to_user(..., link, len);

        (inlined in vfs_readlink)
        generic_readlink(dentry, ...):
            struct inode *inode = d_inode(dentry);
            const char *link = inode->i_link;
            ...
            readlink_copy(..., link);

In support of usercopy hardening, this patch defines a region in the
exofs_inode_cache slab cache in which userspace copy operations are
allowed.

This region is known as the slab cache's usercopy region. Slab caches
can now check that each dynamically sized copy operation involving
cache-managed memory falls entirely within the slab's usercopy region.

This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <dave@nullcore.net>
[kees: adjust commit log, provide usage trace]
Cc: Boaz Harrosh <ooo@electrozaur.com>
Signed-off-by: Kees Cook <keescook@chromium.org>